[EditLine] Rewrite GetHistoryFilePath
Rewrite the GetHistoryFilePath implementation without relying on
FileSpec in the spirit of our discussion in D61994.
It changes LLDBs behavior in two ways:
1. We now only use the -widehistory suffix when LLDB is built with wchar
support, instead of as the fallback from when the ~/.lldb directory
isn't writable.
2. When the ~/.lldb directory isn't writable, we don't write any history
files at all. Previously we would write them to the user's home
directory (with the incorrect wide suffix), polluting ~ with a
different file for every IO handler.
Differential revision: https://reviews.llvm.org/D62216
llvm-svn: 361412